6 Error Recovery

It is not usually acceptable to have a program terminate on a syntax
error. For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should accept
another expression.

In a simple interactive command parser where each input is one line, it may
be sufficient to allow yyparse to return 1 on error and have the
caller ignore the rest of the input line when that happens (and then call
yyparse again). But this is inadequate for a compiler, because it
forgets all the syntactic context leading up to the error. A syntax error
deep within a function in the compiler input should not cause the compiler
to treat the following line like the beginning of a source file.

You can define how to recover from a syntax error by writing rules to
recognize the special token error. This is a terminal symbol that
is always defined (you need not declare it) and reserved for error
handling. The Bison parser generates an error token whenever a
syntax error happens; if you have provided a rule to recognize this token
in the current context, the parse can continue.

For example:

stmts:
%empty
| stmts '\n'
| stmts exp '\n'
| stmts error '\n'

The fourth rule in this example says that an error followed by a newline
makes a valid addition to any stmts.

What happens if a syntax error occurs in the middle of an exp? The
error recovery rule, interpreted strictly, applies to the precise sequence
of a stmts, an error and a newline. If an error occurs in
the middle of an exp, there will probably be some additional tokens
and subexpressions on the stack after the last stmts, and there
will be tokens to read before the next newline. So the rule is not
applicable in the ordinary way.

But Bison can force the situation to fit the rule, by discarding part of
the semantic context and part of the input. First it discards states
and objects from the stack until it gets back to a state in which the
error token is acceptable. (This means that the subexpressions
already parsed are discarded, back to the last complete stmts.)
At this point the error token can be shifted. Then, if the old
lookahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable. In
this example, Bison reads and discards input until the next newline so
that the fourth rule can apply. Note that discarded symbols are
possible sources of memory leaks, see Freeing
Discarded Symbols, for a means to reclaim this memory.

The choice of error rules in the grammar is a choice of strategies for
error recovery. A simple and useful strategy is simply to skip the rest of
the current input line or current statement if an error is detected:

stmt: error ';' /* On error, skip until ';' is read. */

It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed. Otherwise the
close-delimiter will probably appear to be unmatched, and generate another,
spurious error message:

primary:
'(' expr ')'
| '(' error ')'
…
;

Error recovery strategies are necessarily guesses. When they guess wrong,
one syntax error often leads to another. In the above example, the error
recovery rule guesses that an error is due to bad input within one
stmt. Suppose that instead a spurious semicolon is inserted in the
middle of a valid stmt. After the error recovery rule recovers
from the first error, another syntax error will be found straightaway,
since the text following the spurious semicolon is also an invalid
stmt.

To prevent an outpouring of error messages, the parser will output no error
message for another syntax error that happens shortly after the first; only
after three consecutive input tokens have been successfully shifted will
error messages resume.

Note that rules which accept the error token may have actions, just
as any other rules can.

You can make error messages resume immediately by using the macro
yyerrok in an action. If you do this in the error rule’s action, no
error messages will be suppressed. This macro requires no arguments;
‘yyerrok;’ is a valid C statement.

The previous lookahead token is reanalyzed immediately after an error. If
this is unacceptable, then the macro yyclearin may be used to clear
this token. Write the statement ‘yyclearin;’ in the error rule’s
action.
See Special Features for Use in Actions.

For example, suppose that on a syntax error, an error handling routine is
called that advances the input stream to some point where parsing should
once again commence. The next symbol returned by the lexical scanner is
probably correct. The previous lookahead token ought to be discarded
with ‘yyclearin;’.

The expression YYRECOVERING () yields 1 when the parser
is recovering from a syntax error, and 0 otherwise.
Syntax error diagnostics are suppressed while recovering from a syntax
error.